In a direct injection (DI) diesel engine, there is often a significant spread in the fuel injector hardware from cylinder-to-cylinder. This causes significant variations in how much fuel is actually injected in the various engine cylinders. There are also significant variations in how much recirculated exhaust gas (EGR) is trapped in the various cylinders due to variations in EGR distribution in the intake manifold. This results in variations in ignition delay for the various cylinders which, in turn, cause combustion events to be timed and phased differently between the cylinders. These differences result in producing different work outputs or indicated mean effective pressures (IMEP) from the various cylinders, as well as differing levels of emission products, such as nitrogen oxides (NOx), particulates (PM), hydrocarbons (HC) and carbon monoxide (CO), etc.
There are also variations in actual compression ratio between cylinders which cause differences in efficiency of combustion and thus a difference in IEMP produced by each cylinder. In addition, fuel quality differences from one fill-up to the next also may cause the engine to run differently.
Current state-of-the-art diesel engine controls try to address these problems by calibrating the engine actuators (injection timing, EGR and turbo charger boost pressure) for the worst cylinder and by having significant engineering margins in the cylinder calibration. IMEP/torque differences between cylinders due to the injectors are also addressed at idle by analyzing engine speed fluctuations at the relatively constant idle operating point. State-of-the-art controls do not have the capability to identify the complex combinations of different sources of variation causing cylinders to produce varying power and emissions. Thus, if EGR differences or air differences are causing smoke and NOx differences, and also cause IMEP's to differ in various cylinders, injector fuel quantity trims will be used to fix the problem, although that is not the source of the problem. Similarly, if fuel quantity differences cause variations in ignition delay, only timing will be used to adjust for the start of ignition, although that is not the problem.